This invention relates to transducers and more particularly to broadband phased array transducers for use in the medical diagnostic field.
Ultrasound machines are often used for observing organs in the human body. Typically, these machines contain transducer arrays for converting electrical signals into pressure waves. Generally, the transducer array is in the form of a hand-held probe which may be adjusted in position to direct the ultrasound beam to the region of interest. Transducer arrays may have, for example, 128 transducer elements for generating an ultrasound beam. An electrode is placed at the front and bottom portion of the transducer elements for individually exciting each element, generating pressure waves. The pressure waves generated by the transducer elements are directed toward the object to be observed, such as the heart of a patient being examined. Each time the pressure wave confronts tissue having different acoustic characteristics, a wave is reflected backward. The array of transducers may then convert the reflected pressure waves into corresponding electrical signals. An example of a previous phased array acoustic imaging system is described in U.S. Pat. No. 4,550,607 granted Nov. 5, 1985 to Maslak et al. and is incorporated herein by reference. That patent illustrates circuitry for combining the incoming signals received by the transducer array to produce a focused image on the display screen.
Broadband transducers are transducers capable of operating at a wide range of frequencies without a loss in sensitivity. As a result of the increased bandwidth provided by broadband transducers, the resolution along the range axis may improve, resulting in better image quality.
One possible application for a broadband transducer is contrast harmonic imaging. In contrast harmonic imaging, contrast agents, such as micro-balloons of protein spheres, are safely injected into the body to illustrate how much of a certain tissue, such as the heart, is active. These micro-balloons are typically one to five micrometers in diameter and, once injected into the body, may be observed via ultrasound imaging to determine how well the tissue being examined is operating. Contrast harmonic imaging is an alternative to Thallium testing where radioactive material is injected into the body and observed by computer generated tomography. Thallium tests are undesirable because they employ potentially harmful radioactive material and typically require at least an hour to generate the computer image. This differs from contrast harmonic imaging in that real-time ultrasound techniques may be used in addition to the fact that safe micro-balloons are employed.
In B. Schrope et al., "Simulated Capillary Blood Flow Measurement Using a Nonlinear Ultrasonic Contrast Agent," Ultrasonic Imaging, Vol. 14 at 134-58 (1992), which is incorporated herein by reference, Schrope discloses that an observer may clearly see the contrast agents at the second operating harmonic. That is, at the fundamental harmonic, the heart and muscle tissue is clearly visible via ultrasound techniques. However, at the second harmonic, the observer is capable of clearly viewing the contrast agent itself and thus may determine how well the respective tissue is performing.
Because contrast harmonic imaging requires that the transducer be capable of operating at a broad range of frequencies (i.e. at both the fundamental and second harmonic), existing transducers typically cannot function at such a broad range. For example, a transducer having a center frequency of 5 Megahertz and having a 70% ratio of bandwidth to center frequency has a bandwidth of 3.25 Megahertz to 6.75 Megahertz. If the fundamental harmonic is 3.5 Megahertz, then the second harmonic is 7.0 Megahertz. Thus, a transducer having a center frequency of 5 Megahertz would not be able to adequately operate at both the fundamental and second harmonic.
In addition to having a transducer which is capable of operating at a broad range of frequencies, two-dimensional transducer arrays are also desirable to increase the resolution of the images produced. An example of a two-dimensional transducer array is illustrated in U.S. Pat. No. 3,833,825 to Haan issued Sep. 3, 1974 and is incorporate herein by reference. Two-dimensional arrays allow for increased control of the excitation of ultrasound beams along the elevation axis, which is otherwise absent from conventional single-dimensional arrays. However, two-dimensional arrays are also difficult to fabricate because they typically require that each element be cut into several segments along the elevation axis, connecting leads for exciting each of the respective segments. A two-dimensional array having 128 elements in the azimuthal axis, for example, would require at least 256 segments, two segments in the elevation direction, as well as interconnecting leads for the segments. In addition, they require rather complicated software in order to excite each of the several segments at appropriate times during the ultrasound scan because there would be at least double the amount of segments which would have to be individually excited as compared with a one-dimensional array.
Further, typical prior art transducers having parallel faces relative to the object being examined tend to produce undesirable reflections at the interface between the transducer and object being examined, producing what is called a "ghost echo." These undesirable reflections may result in a less clear image being produced.